Skin-effect corrections are applied to measurements made by a transverse induction logging tool to give corrected measurements indicative of vertical conductivities of a formation. Here, the threshold value Vth of the comparator is continuously increased in response to a period in which the sine-wave output signal Sp is greater than the threshold value Vth, and is continuously decreased in response to a period in which the sine-wave output signal Sp is smaller than the threshold value Vth. The internal contact portion has a taper shape which expands towards the lower end from the upper end, and is formed so as to contact with the internal surface of the external cylinder portion and conduct to the external cylinder portion. Related methods are also discussed.